Abstract: Stratospheric Aerosol Injection (SAI) aims to mitigate climate change by releasing aerosols into the stratosphere to reflect incoming shortwave radiation. The radiative efficiency of SAI (i.e., the ratio of injected mass flux to radiative forcing) depends strongly on the stratospheric lifetime of injected particles. In this study, we use a Lagrangian trajectory model (LAGRANTO), modified to account for sedimentation, to analyze the sensitivity of particle lifetime to injection locations.

For the first time, we find that choosing injection longitude could notably increase particle lifetime in the stratosphere, especially for injections below 20 km. For example, the particles injected over the Indian Ocean (60° E to 105° E) in winter have a mean lifetime of 1.33 years, which is 23% larger than particles injected at the same altitude and latitude range (18 km, 10° S to 10° N) over the East Pacific (75° W to 120° W).

We explore four injection strategies to maximize particle lifetime in the stratosphere by selecting injection locations. Selecting injection latitude and longitude can help to achieve a lower injection altitude (more than 1 km lower) without sacrificing lifetime. For example, a uniform injection in the tropical area at 20 km has a mean particle lifetime of 2.0 years, we can select the injection latitude and longitude to lower the injection altitude by 1.5 km (at 18.5 km) to achieve a similar mean lifetime (i.e., 2.0 years). Because maximizing particle lifetime by selecting injection location will increase the interhemispheric imbalance, we designed an injection strategy that can maximize the particle lifetime in the stratosphere subjecting to the interhemispheric balance constraint.

Our results complement SAI studies with GCMs to inform future injection strategy design. The modified LAGRANTO model uses 3-hourly ERA5 data as input, which provides a better estimate of stratospheric transport than GCMs. But the LAGRANTO model cannot model aerosol dynamics nor the response of the climate to SAI.